Segmentation is the process that produces a metameric pattem of somites during embryonic development. This segmental pattern of organization is critical for the subsequent development of structures such as the vertebral column, associated spinal nerves, muscles and other tissues. Defects in this process during development leads to vertebral column abnormalities, such as that observed in spondylocostal dysostosis syndrome in humans. A key process that is required for this segmental patterning is a segmentation clock that is mediated by the Notch pathway. Expression of genes in this pathway oscillate on and off during each segmental cycle, specifying the boundaries between each segment by regulating the segmental expression of transcription factors. The key role of the Notch pathway in this process is indicated in part by the observation that the spondylocostal dysostosis syndrome in humans is linked to a haploinsufficiency in the human Delta3-1ike gene, which encodes a ligand in the Notch pathway. The goal of this application is to determine the molecular bases of the segmentation clock. The experiments proposed will dissect the genetic regulatory elements that are required for generating oscillations in the expression of Notch pathway genes in Xenopus embryos where segmentation occurs. Other experiments will examine how the output of these oscillations regulate segmental gene expression. Finally experiments are proposed to identify small molecules that disrupt these oscillations, as well as other genes required for the oscillations to occur. Results from these experiments will generate basic knowledge about a developmental mechanism that is key to the formation of the vertebrate body plan, ir_cluding that of humans.